The heterogeneity of leukemias has been unmasked by probes designed to identify proteins in individual cells. This experimental approach has led to major advances in the study of differentiation in normal hematopoiesis. We have purified, characterized, and prepared antibodies to three human enzymes that are markers of lymphoid differentiation. We will now pursue biochemical questions about protein processing and structural relationships of these proteins to tumor antigens and to other enzymes. Specific aims include: (1)\continuing to develop monoclonal antibodies to TdT and ADA. The monoclonal antibodies will be used to develop immunoperoxidase methods of protein detection and will be used for the construction of affinity columns for rapid protein purifications. The antibodies will also be used in binding studies to dissect the structures of multiple forms of the two proteins and to assess structural relationships with other proteins; (2)\continuing the significant progress made in cloning a cDNA coding for human ADA. When the probe is isolated, we will use it to study levels of mRNA coding for ADA and will begin studies to isolate the human ADA gene. As a part of this work, protein sequencing studies will be continued to obtain sequence data of the N and C termini of human ADA; and (3)\beginning studies to purify human MTA-Pase to homogeneity. (B)